matrix-spec-proposals/proposals/3989-redact-origin-field.md

MSC3989: Redact origin property on events

The current redaction algorithm keeps the top-level origin property on events during redaction, however, as of this writing, the only use within the spec of origin as a top-level event property is a malformed example of event format. The property has no significant meaning in modern room versions.

Within the ecosystem, it's clear that we'd prefer the property to disappear, and have tried to do so in the past. The malformed examples are even known to us.

What's not clear, and mentioned in a comment, is whether the origin property is actually used. There do not appear to be any auth rules or similar which would use the property, however it'd hardly be the first time that the spec was wrong about an ancient room version like v1. What is clear is that Synapse, where this question would be asked, wants to drop support for the property and has taken steps towards that mission by fixing bugs in the area. In a quick audit of the Synapse codebase during implementation of this MSC, the origin property appears unused.

Given the above context, this proposal removes the origin property from the redaction algorithm in a future room version, leaving it as-is for existing versions (not that an MSC can change the behaviour of an already-stable room version anyways).

Some other properties are additionally useless in modern room versions, however they are already adapted by MSC2176.

Proposal

In a future room version, the origin property is removed from the list of event keys which are kept during redaction. Note that this requires a new room version because changing the redaction algorithm changes how event IDs are calculated, as they are reference hashes which redact the event during calculation.

Potential issues

No major concerns.

Alternatives

No significant alternatives.

Security considerations

No major concerns.

Unstable prefix

While this MSC is not considered stable, implementations should use org.matrix.msc3989 as the room version identifier, using v10 as a base.

Dependencies

No blocking dependencies.

This MSC would partner well with the following MSCs, however:

• MSC2176
• MSC2175
• MSC2174
• MSC3821